1. Field of the Invention
The present invention relates to a lithium-ion capacitor including a positive electrode, a negative electrode and an aprotic organic solvent electrolytic solution of a lithium salt as an electrolytic solution.
The lithium-ion capacitor of the present invention is considerably effective when it is used for a driving source and an auxiliary electricity storage source in electric vehicles, hybrid electric vehicles and the like. The lithium-ion capacitor may also be used for driving electricity storage sources of power-assisted bicycles and powered wheel chairs, storage devices for storing various types of energies in solar power systems and wind power systems etc., and electricity storage sources for home-use electric appliances.
2. Description of the Related Art
A so-called lithium-ion secondary battery is known in which a carbon material such as graphite is used for a negative electrode and a lithium-containing metal oxide such as LiCoO2 is used for a positive electrode. The lithium-ion secondary battery, which has a high capacity and is a prevailing electricity storage device, has practically been used primarily for main power sources of note personal computers and mobile phones. The lithium-ion secondary battery is a so-called rocking-chair type battery in which after the battery is assembled, the lithium-containing metal oxide of the positive electrode supplies lithium ions to the negative electrode by charging the battery, and when it is discharged, the lithium ions are returned from the negative electrode to the positive electrode. The lithium-ion secondary battery has the advantageous features of high voltage, high capacity and high safety.
In current circumstances of growing interest in environmental issues, the electricity storage devices (main power source and auxiliary power source) for the electric vehicles and the hybrid electric vehicles, which will supersede gasoline-powered vehicles, are vigorously developed. A lead battery has been used for the battery for the automobile. Recently, electrical systems and devices that are installed and assembled into the automobile have been functionally enhanced and increased in number. To secure satisfactory operations of them, it is required to increase the energy density and the output density. In this circumstance, the market demands new electricity storage devices so improved as to be able to produce such energy and output densities.
Attention has been given to the lithium ion secondary and electric double layer capacitor as such new electricity storage devices. The lithium-ion secondary battery is advantageous in that it has high energy density but is disadvantageous in that its output characteristics and safety are still uncertain and its cycle life is not long. The electric double layer capacitor, which is now used for memory backup power sources in IC and LSI, has a defect that its discharging capacity per charge is smaller than of the battery. However, the electric double layer capacitor has excellent features of high output characteristic and maintenance free, which are not possessed by the lithium-ion capacitor. The electric double layer capacitor is excellent in instantaneous charging/discharging characteristic and endures several tens thousands cycles of charging/discharging operations.
The electric double layer capacitor has the advantages just mentioned. However, the energy density of an ordinary electric double layer capacitor in the prior art is about 3 to 4 Wh/L, two orders of magnitude lower than the lithium-ion secondary battery. For use with the electric vehicles, it is said that 6 to 10 Wh/L is required for practical use, and 20 Wh/L is required for the spread of this storage device.
The new electricity storage device, also called a hybrid capacitor, has attract an attention as the electricity storage device satisfying the requirements of the high energy density and high output characteristic, in recent years. The hybrid capacitor is a combination of the lithium-ion secondary battery technology and the electricity storage principle of the electric double layer capacitor. In the common hybrid capacitor, a positive electrode is a polarizable electrode and a negative electrode is a nonpolarizable electrode. It has attracted an attention as the electricity storage device having the high energy density of the battery and high output characteristics of the electric double layer capacitor. There is a proposal of another hybrid capacitor in which a negative electrode capable of occluding and desorbing lithium ions is brought into contact with metal lithium to lower a potential of a negative electrode by chemically or electrochemically occluding or carrying (to be referred to also as “doping”) lithium ions, whereby the withstand voltage is increased and the energy density is remarkably increased. The new technology is disclosed in JP-A-8-107048, 9-55342, 9-232190 and 11-297578, and International Publication WO98/033227.
This type of the hybrid capacitor is expected to have high performances, but it has the following problems. To dope the negative electrode with lithium ions, much time is needed. Further, it is difficult to uniformly dope the entire negative electrode with the lithium ions. It is generally understood that it is difficult to realize the hybrid capacitor in practical levels in large-capacity cells, such as a cylindrical device having wound electrodes and a square battery having a lamination of electrodes.
The problem was successfully solved. This successful technology follows. Through holes are formed in a negative electrode current collector and a positive electrode current collector, which constitute a cell, such that the through holes pass through those electrodes. Lithium ions are moved through the through holes, and at the same time, the metal lithium as a lithium ion supply source and the negative electrode are shortcircuited. By placing the metal lithium at the end of the cell, the negative electrode in the cell is entirely doped with the lithium ions (International Publication WO98/033227). Usually, the negative electrode is doped with lithium ions. However, the International Publication WO98/033227 describes that the same effect is produced when the negative electrode and the positive electrode are doped with lithium ions, and the positive electrode in lieu of the negative electrode is doped with lithium ions.
Thus, even in the large-capacity cells, such as the cylindrical device having wound electrodes and the square battery having a lamination of electrodes, the entire negative electrode of the device can be uniformly doped with the lithium ions for a short time. With the increased withstand voltage, the energy density of the capacitor is remarkably increased, realizing of the capacitors having high capacity and large output density originally possessed by the electric double layer capacitor is in sight.
To put the capacitor of high capacity in practical use, it is required to further increase the capacity, energy density, and output density, and to secure high durability.